an alloy of these metals. Sulphur compounds are often formed on the surface, when the above metals are exposed for some time to the atmosphere of large towns or rooms where coal-gas is burned. These films may be removed by immersion in suitable acid dips. For this purpose a series of liquids is used: pickle or spent aquafortis is very generally employed for a preliminary dip. The articles are allowed to remain in it until the scale of oxide has disappeared, leaving, after rinsing, a uniform metallic lustre. Dipping in old aquafortis is recommended for two reasons: it economises the cost of new acid, and, as its action is slow, it prevents the too rapid corrosion of the cleansed copper during the time of the solution of the protoxide. A dipping liquid may consist of a mixture of 64 parts commercial sulphuric acid, 32 parts of aquafortis, 1 part hydrochloric acid, and 64 parts water. § 62B. Copper, brass, bronze, German silver, etc., are often cleaned by heating them to dull redness, and then plunging into dilute sulphuric acid. (Those having solder upon them are not heated thus; neither are articles of castbronze, because they would be liable to crack.) They are then soaked in old aquafortis, until, after rinsing, they look uniformly metallic; they may then be dipped in strong aquafortis for a few seconds and swilled. The strawcoloured aquafortis acts the best; the white variety acts too feebly, and the red too strongly. It is best to use the dips cold, and to have a considerable bulk of liquid to prevent them becoming too hot by the immersion of the heated metal. In diluting strong sulphuric acid with water a considerable amount of heat is generated by the chemical action which takes place between them, and if the mixture is made too rapidly the vessel which contains it is liable to be cracked and the liquid to be projected on to the operator. The acid should always be poured into the water and not vice versa. To dip gilding metal bright: Immerse it in weak aquafortis until there is a black scale formed, then dip it in strong pickle for a few minutes (N.B.-Strong pickle is partially exhausted aquafortis; weak pickle is the same diluted with the washings), then dip it quickly into aquafortis, then into several wash-waters in succession. There are various mixtures which may be employed for imparting a bright lustre to brass, German silver, etc., by dipping; the following is one of them:-1 measure of nearly exhausted aquafortis, 2 of water, and 6 of hydrochloric acid; the articles should be immersed in it a few minutes, or until, after washing off the black mud which entirely covers them, they look bright; they are then cleaned and dipped again. It is convenient for removing the sand, etc., which adheres to castings. Large articles may remain in this bath for twenty to thirty minutes. § 62c. Dipping in Aquafortis, Common Salt, and Soot. -Brass and similar articles, after cleaning in pickle, are rinsed in water, well shaken and drained, then dipped in a bath consisting of 100 parts nitric acid, 1 part of common salt, and 1 part of calcined soot. This mixture attacks the metal with great energy, and therefore it should only remain in it a few seconds. The volume of acid should be twenty times that of the articles immersed in it to prevent undue heating and too rapid weakening of the acid. When removed, the articles should be quickly rinsed in water to prevent the production of nitrous fumes. They then present a fine lustre varying from red to golden-yellow and greenish-yellow, according to the composition of the alloy. If the metal is not swilled in water after removing from the acid, there rises on its surface a green froth, and nitrous vapours are given off which indicate the decomposition of the acid with which the metal is covered. When the vapours have disappeared the metal remains dull black, even after swilling. This last mode of operating, called blacking by aquafortis, is used by some colourers to give a dull dark colour to brass and bronze work. Aquafortis is spent when its action on copper alloys is too slow, and when the objects removed from the bath are covered with a bluish-white film. Such acid is termed "pickle," and is used for the preliminary cleaning, or for forming what is termed a whitening bath. Very good aquafortis may appear too weak and cleanse imperfectly by dipping when the temperature is too high, or too low, as in the case of frosty weather. § 62D. Whitening Bath.—This consists of old aquafortis, sulphuric acid, common salt, and raw soot. Pour into a stoneware vessel a certain quantity of old aquafortis, and add twice the volume of commercial sulphuric acid. Allow the mixture to stand till the next day. The copper nitrate of the old aquafortis is converted into copper sulphate, which crystallises against the sides of the vessel. Decant the clear liquid into another vessel and add 2 to 3 per cent of common salt, and an equal quantity of calcined soot. This mixture is less active than the acids used for a bright lustre. The bath may be strengthened when necessary by the addition of aquafortis and sulphuric acid. § 62E. Another dipping liquid may be made with equal parts of aquafortis and sulphuric acid mixed with forty times their bulk of water and allowed to cool; then adding a quantity of common salt equal to about one-fifth that of the strong acid present. Or the following may be used : 1 lbs. nitric acid, 10 grains common salt. To the above ingredients add a mixture of the following if a dead surface is desired : The longer the articles remain in this dip the deader will be the surface. They are then thoroughly swilled and dried as quickly as possible. Or previous to swilling with water they may be momentarily dipped in the bright dipping liquid. Mr. Aitkin says that "dead dipping" was discovered in the following way :-In the year 1832 a dipper in the employ of Mr. David Malins of Birmingham left throughout the night a quantity of articles in the pickle, and when he attempted to produce the bright appearance in the bright dip they presented a dull, frosted yellow surface. Charmed with the effect, certain portions were burnished and the whole lacquered. Acting on the accidental hint, dead dipping was originated, and has now become the recognised mode of finish for brass work generally. Another liquid for dead dipping may be made of— 1 volume of a concentrated solution of potassium bichromate, 2 volumes of concentrated hydrochloric acid. The articles should be left in this solution for some hours, then well swilled in several wash-waters. If, however, they are left exposed to the air for some time without lacquering or further treatment, they become coated with a film of oxide. Dead-dipped articles, while waiting to be bronzed or lacquered, may be kept from oxidising by immersing in clean water, to which half its volume of alcohol has been added. In the case of copper alloys, such as brass, the surface colour will depend not only on the original composition of the alloy, but also on the length of time it has been exposed to the action of the acid. The zinc is oxidised more rapidly than the copper, so that the effect of dipping in aquafortis or other oxidising liquid is to increase the relative quantity of copper on the surface, and to give to the alloy a richer appearance and a deeper colour. When it is desired to clean very small articles and not to appreciably alter the composition, they may be dipped in a solution of 5 parts potassium cyanide dissolved in 95 parts of water. If the coloured brass articles show a granular appearance on the surface after dipping they should be immersed for twelve hours in a mixture of 1 volume of nitric acid, 1 volume of sulphuric acid, and 8 volumes of water. The greyish-black deposit is washed off with water, leaving an agreeable moiré appearance. The articles are next immersed in one of the bright dips above described, then passed through a weak solution of caustic soda, or milk of lime, well washed in water, and dried out in sawdust. If an article remains too long in the bright dip, after being made dull in the dead dip, the dead lustre disappears. If the bath for giving a bright lustre is not available, the objects, after rinsing, may be rapidly passed through the dead dip to remove the dulness of the lustre caused by too long immersion. After long use the compound acids used for bright dipping will give a dead appearance to brasswork. For large embossed work a hot bath for dead lustre is used, composed of-Old aquafortis, 4 to 5 parts; sulphuric acid, 1 part; zinc sulphate, 8 to 10 per cent. More zinc sulphate is added when required for increasing the dulness of the lustre. The lustre, however, after rinsing the article, and passing it through the same bath for one or two seconds, and well swilling, becomes clearer. It is For the production of a granular appearance on brass, etc., a mixture of one part of a saturated solution of potassium bichromate in water, and two parts of concentrated hydrochloric acid, may be employed. The metal is left for some hours until the desired granular effect is produced. then removed and well swilled with water. The operation may be considerably hastened by the aid of an electric current, attaching the metal article to the positive pole of the battery and using a brass plate as the cathode. The liquid for this method may be a very dilute solution of sulphuric and nitric acids, or of potassium bichromate and hydrochloric acid. |